The present invention concerns a resistive mixer for mixing or detecting an electrical or electromagnetic high frequency signal.
The terahertz frequency range or submillimeter wavelength range which is roughly defined as being between 100 gigahertz (GHz) and 10 terahertz (THz) is one of the last ‘dark’ areas in the electromagnetic spectrum. In that frequency range technically usable, in particular coherent sources and corresponding detectors are hitherto not commercially available or are commercially available only at low frequencies. The developments in the last decades have resulted in systems which by virtue of their complexity however are hitherto used only in experimentally distinguished areas such as radio astronomy or atmospheric research. Hitherto inexpensive sources and detectors are not available for applications in everyday life, that being although the THz frequency range has intrinsic advantages over other frequency bands in the electromagnetic spectrum:                many optically opaque materials are transparent in the THz frequency range,        THz radiation is non-ionising and is therefore deemed to be safe in the biomedical field,        given rotatory, vibronic or libratory molecular excitations have a resonance frequency in the THz frequency range,        THz radiation affords essential items of information about charge carrier dynamics, in particular in nanostructures, which play an essential part in future photonic and electronic components,        THz radiation exhibits a lesser degree of scatter compared to optical frequencies and is therefore suitable in particular for use in industrial environments in which for example increased dust formation is involved, and        if communication systems are considered higher frequencies permit greater transmission band widths.        
Most purely electronic apparatuses operating in the THz frequency range are based on GaAs or InP semiconductor technology. It was shown in the end that SiGe and CMOS semiconductor technologies also result in apparatuses operating at up to 100 GHz. At higher frequencies up to 1 THz and above more complex quantum cascade laser systems are used equally as sources as optoelectronic systems based on femtosecond short pulse lasers or mixing of two continuous-wave laser sources.
At the present time the THz radiation is detected with heterodyne mixers, for example Schottky diode mixers, photoconductive detectors or power detectors such as for example photovoltaic detectors, bolometers or Golay cells.
All the above-described technologies however involve considerable complexity in the source and detector components themselves as well as the manufacture thereof so that although they are admittedly used in the field of research and development and in research-related fields of application such as radio astronomy, they are not suitable for mass markets.
U.S. Pat. No. 4,647,848 discloses a field effect transistor circuit which is used as a discrete resistive mixer. The field effect transistor is used to detect the power level of an electromagnetic high frequency signal VRF. A detector circuit from the state of the art is shown in FIG. 1a) of that application. The high frequency signal is coupled into the gate G and by way of a capacitor CGD into the drain D of the field effect transistor FET, wherein the gate G is biased with a dc voltage VG corresponding to the threshold value of the transistor FET. The resulting current at the source N then contains a dc component Ids which is proportional to the square of the amplitude of the high frequency signal. The output signal can be detected as a voltage Vds by way of a corresponding external resistor. In that respect the described receiver circuit is made up of discrete components. The output signal is filtered with a low pass filter to suppress the ac components.
In that respect the operating frequency of the power detector described in U.S. Pat. No. 4,647,848 is limited to a few gigahertz (GHz) by the discrete arrangement and in particular by the predetermined gate length of the field effect transistor.
In comparison the object of the present invention is to provide a resistive mixer which operates at high gigahertz and terahertz frequencies.